Changing field::field_name and Item::name to LEX_CSTRING

Benefits of this patch:
- Removed a lot of calls to strlen(), especially for field_string
- Strings generated by parser are now const strings, less chance of
  accidently changing a string
- Removed a lot of calls with LEX_STRING as parameter (changed to pointer)
- More uniform code
- Item::name_length was not kept up to date. Now fixed
- Several bugs found and fixed (Access to null pointers,
  access of freed memory, wrong arguments to printf like functions)
- Removed a lot of casts from (const char*) to (char*)

Changes:
- This caused some ABI changes
  - lex_string_set now uses LEX_CSTRING
  - Some fucntions are now taking const char* instead of char*
- Create_field::change and after changed to LEX_CSTRING
- handler::connect_string, comment and engine_name() changed to LEX_CSTRING
- Checked printf() related calls to find bugs. Found and fixed several
  errors in old code.
- A lot of changes from LEX_STRING to LEX_CSTRING, especially related to
  parsing and events.
- Some changes from LEX_STRING and LEX_STRING & to LEX_CSTRING*
- Some changes for char* to const char*
- Added printf argument checking for my_snprintf()
- Introduced null_clex_str, star_clex_string, temp_lex_str to simplify
  code
- Added item_empty_name and item_used_name to be able to distingush between
  items that was given an empty name and items that was not given a name
  This is used in sql_yacc.yy to know when to give an item a name.
- select table_name."*' is not anymore same as table_name.*
- removed not used function Item::rename()
- Added comparision of item->name_length before some calls to
  my_strcasecmp() to speed up comparison
- Moved Item_sp_variable::make_field() from item.h to item.cc
- Some minimal code changes to avoid copying to const char *
- Fixed wrong error message in wsrep_mysql_parse()
- Fixed wrong code in find_field_in_natural_join() where real_item() was
  set when it shouldn't
- ER_ERROR_ON_RENAME was used with extra arguments.
- Removed some (wrong) ER_OUTOFMEMORY, as alloc_root will already
  give the error.

TODO:
- Check possible unsafe casts in plugin/auth_examples/qa_auth_interface.c
- Change code to not modify LEX_CSTRING for database name
  (as part of lower_case_table_names)

sql/sql_statistics.h

@@ -94,12 +94,12 @@ int alloc_statistics_for_table_share(THD* thd, TABLE_SHARE *share,
                                      bool is_safe);
 int alloc_statistics_for_table(THD *thd, TABLE *table);
 int update_statistics_for_table(THD *thd, TABLE *table);
-int delete_statistics_for_table(THD *thd, LEX_STRING *db, LEX_STRING *tab);
+int delete_statistics_for_table(THD *thd, LEX_CSTRING *db, LEX_CSTRING *tab);
 int delete_statistics_for_column(THD *thd, TABLE *tab, Field *col);
 int delete_statistics_for_index(THD *thd, TABLE *tab, KEY *key_info,
                                 bool ext_prefixes_only);
-int rename_table_in_stat_tables(THD *thd, LEX_STRING *db, LEX_STRING *tab,
-                                LEX_STRING *new_db, LEX_STRING *new_tab);
+int rename_table_in_stat_tables(THD *thd, LEX_CSTRING *db, LEX_CSTRING *tab,
+                                LEX_CSTRING *new_db, LEX_CSTRING *new_tab);
 int rename_column_in_stat_tables(THD *thd, TABLE *tab, Field *col,
                                   const char *new_name);
 void set_statistics_for_table(THD *thd, TABLE *table);
@@ -259,10 +259,10 @@ static inline
 int rename_table_in_stat_tables(THD *thd, const char *db, const char *tab,
                                 const char *new_db, const char *new_tab)
 {
-  LEX_STRING od= { const_cast<char*>(db), strlen(db) };
-  LEX_STRING ot= { const_cast<char*>(tab), strlen(tab) };
-  LEX_STRING nd= { const_cast<char*>(new_db), strlen(new_db) };
-  LEX_STRING nt= { const_cast<char*>(new_tab), strlen(new_tab) };
+  LEX_CSTRING od= { db, strlen(db) };
+  LEX_CSTRING ot= { tab, strlen(tab) };
+  LEX_CSTRING nd= { new_db, strlen(new_db) };
+  LEX_CSTRING nt= { new_tab, strlen(new_tab) };
   return rename_table_in_stat_tables(thd, &od, &ot, &nd, &nt);
 }